Soluble titanium compounds are of great importance in chemistry and related technical fields. For example, they can be used as versatile reagents in chemical synthesis and analysis. Furthermore, of particular importance is the use of these compounds for the precipitation of titanium dioxide in or from solutions, for example in the form of sols or gels, of finely divided powders, as thin coatings on any substrates, for example on glass for optical or decorative purposes or in the preparation of pearl luster pigments based on mica coated with TiO.sub.2. In the high-performance ceramics sector, the element titanium plays a widespread and often essential role in functional ceramics, in particular in electroceramics and piezo-ceramics
For the above applications, titanyl nitrate or aqueous titanyl nitrate solution is the preferred titanium source.
Other soluble or liquid titanium compounds such as titanyl sulphate (TiOSO.sub.4), titanium tetrachloride and titanyl chloride (TiOCl.sub.2) are unsuitable as such for ceramic purposes. Organic titanium compounds such as, for example, orthotitanates are expensive. All these compounds have in common that owing to their high sensitivity to hydrolysis they are very unstable and difficult to handle.
The key substance as a source for virtually all significant titanium compounds is titanium tetrachloride, which is prepared from titanium dioxide. The latter is in turn extracted from naturally occurring minerals.
Astonishingly, the relevant technical literature gives hardly any directions for a preparation of titanyl nitrate or its aqueous solution which is practical and can be readily carried out on an industrial scale.
Theoretically, it should be possible to prepare titanyl nitrate in aqueous solution from titanium tetrachloride or its partial hydrolysis product titanyl chloride by reaction with nitric acid in accordance with the equations EQU TiCl.sub.4 +2HNO.sub.3 +H.sub.2 O.fwdarw.TiO(NO.sub.3).sub.2 +4HCl
or EQU TiOCl.sub.2 +2HNO.sub.3 .fwdarw.TiO(NO.sub.3).sub.2 +2HCl
In practice, however, reactions based on these equations do not provide the desired objective, since at least partial hydrolysis, usually during the reaction, occurs precipitating TiO.sub.2 and/or TiO.sub.2 hydrogels. A complete redissolution of titanium dioxide or the TiO.sub.2 hydrogels once precipitated is not possible in practice. Furthermore, it is not possible to completely remove the hydrochloric acid formed in the reaction from the reaction solution. Attempts to drive off the hydrochloric acid, for example by heating or passing through inert gas, remain incomplete and likewise lead to precipitation of TiO.sub.2. Precipitation as silver chloride, which is conceivable in principle, is impractical for economic reasons, even for relatively low residual chloride contents.
The presence of chloride is extremely undesirable in high-temperature solid-state reactions such as the sintering of ceramics or the calcination of TiO.sub.2 coatings. Metal chlorides are known to be very volatile at high temperatures. Even very small amounts of chloride in ceramic compositions for high-performance ceramics, therefore, result in composition changes during sintering with, for example, the content of dopants changing dramatically.
An acceptable limit for a residual chloride content in a titanyl nitrate solution which is tolerable for the above objectives can be regarded as about 200 ppm, based on titanyl nitrate.
DE 41 10 685 A1 describes a process for preparing low-chloride aqueous solutions of titanyl nitrate by reaction of titanium tetrachloride or titanyl chloride with nitric acid, wherein the reaction is carried out in the presence of excess nitric acid and/or hydrogen peroxide, by which means the chloride present is oxidized to chlorine and the product obtained has a residual chloride content of less than 200 ppm.
However, this process, which leads to an excellent result per se, has some disadvantages which become unpleasantly noticeable when the reaction is carried out in practice and, in particular, in industry. On the one hand, this process requires the handling of concentrated, in particular, fuming, nitric acid, and highly concentrated hydrogen peroxide. These chemicals are known to be extremely dangerous. Transport, storage and use require very strict safety measures. On the other hand, the reaction forms, apart from chlorine gas, relatively large amounts of nitrous gases which have to be trapped and rendered harmless. Furthermore, the end point of the reaction at which the desirably low chloride content is reached is difficult to determine unless undesirably large excesses of nitric acid or hydrogen peroxide are used.